In industry, coatings are applied to parts and products for various purposes. Certain coatings, for instance, are applied to surfaces to provide low friction, wear resistance, rust resistance, corrosion resistance, toughness, or other specific properties. The types of parts and products to which such coatings are applied can be metal machinery such as gears, drive-shafts, ball valves, etc.
One class of coatings currently used in industry to enhance machine component reliability and reduce cycle-life cost is the engineered tribocoatings. These coatings are either thermally sprayed using chromium or tungsten carbide or are electroplated using hard chrome. While thermal spraying is advantageously a line-of-sight (LOS) process, it can only be used to coat components of simple shapes. In contrast, electroplating of hard chrome is a non-line-of-sight (NLOS) process that can be used to coat complex shapes. Unfortunately, the electroplated coatings can suffer from severe environmental concerns due to the emission of carcinogenic hexavalent chromium ions produced during manufacture.
Black oxide coatings formed by converting existing alloys to a black form of rust are also currently used in industry. Black oxide coatings offer good rust resistance and excellent lubricity in the presence of oil lubricants. Thus, black oxide coatings can be useful in components requiring tight tolerances such as rotary gears. However, such coatings are very soft and loosely bonded to the component substrates. They also offer very limited wear resistance under loss-of-lubricant conditions and very limited corrosion protection when used in corrosive systems.
Iron sulfide lubricant coatings such as FeS are used in industry due to their good lubricating properties, but they also suffer from environmental drawbacks as the manufacturing process produces harmful effluents or emissions. Plasma spray tribological coatings (with or without silver binder) containing lubricating solids distributed throughout a wear-resistant matrix of nickel-cobalt alloy-bonded chromium carbide have been developed for aerospace applications to improve low-temperature friction. These coatings, however, are expensive and lack versatility.
Hard chrome plating is a technique that has been in commercial production for over fifty years. While hard chrome plating provides a protective hard coating on machine parts, it too suffers costly environmental problems including carcinogenic hexavalent chromium ion emissions during the electroplating sequence.